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CAPE CANAVERAL, FLA. (AP) - Half of the International Space Station’s cooling system suddenly shut down during the weekend, forcing the astronauts to power down equipment and face the likelihood of urgent spacewalking repairs.

After huddling Sunday, NASA managers gave preliminary approval for a pair of spacewalks, the first of which would take place later this week. Two of the Americans on board were already scheduled to conduct a spacewalk Thursday for routine maintenance, though the repairs would supersede the original chores.

Officials stressed that the six occupants were in no danger, and that the orbiting complex was in a stable situation.

The trouble arose Saturday night, when one of the two ammonia-fed cooling loops shut down. Alarms sounded throughout the sprawling outpost as the circuit breaker for the pump in that line tripped, causing the pump to stop working.

The cooling system is critical for on-board operations. The two ammonia lines ensure that all the station’s electronic equipment does not overheat and with one of those out of commission, there is no safeguard in case of a second failure.

Flight controllers tried to restart the disabled ammonia pump early Sunday, but the circuit breaker tripped again. No further repair attempts were planned, at least for now. In fact, the astronauts were allowed to sleep in because of all the late-night disruptions.

Any repairs later this week will involve replacing that ammonia pump, a difficult job that would require two spacewalks. Two spare pumps are stored on the outside of the station.

The first repair spacewalk likely would occur Thursday at the earliest, with the second excursion two or three days later. A final decision on whether to proceed with the task will be made following additional engineering review.

Among the equipment powered off for now: the Global Positioning System circuit, several power converters and a set of devices that route commands to various equipment.

Two of the four gyroscopes initially were shut down _ part of the space station’s pointing and navigating system. But the crew installed a jumper cable to bring up a third gyroscope, leaving the station in a much more stable position.

On board are three Americans and three Russians.

No space shuttle visits are planned before November. Only two U.S. shuttle missions remain before the fleet is retired; a third and final flight for next summer is under consideration.